Transmission control circuit



June 11, 1935. B gNsoN 2,004,593

TRANSMISSION CONTROL CIRCUIT Filed Sept. 17, 1951 zsneets-sneet 1 FIG! FIG.)

3 i film? S fl -i-l 8; a. mom/501v A T TORNE Y B. G. BJORNSON TRANSMISSION CONTROL CIRCUIT June 11, 1935.

Filed Sept. 17, 1931 2 Sheets-Shget. 2

AMP.

FIG. 4 U

'INVENTOIP B. a. BJORNSON B) ATTORNEY Patented June 11, 1935 r UNITED PATENT: orrics TRANSMISSION CONTROL CIRCUIT Bjorn G. Bjornson, Brooklyn, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York,' N, Y., a corporation of NewYork Application September- 17, 1931, Serial No. 3,277 9 Claims, (01. 178 -44) This invention relates to circuits for controlfeed-back circuits are preferably of the so-called ling transmission in signaling transmission sysseries type, that'is, the type in which the feedtems, and particularly to wave-controlled circuits back is series conneeteqwi h th gr d circuit of for suppressing echoes and preventing-singing in the var abl im edance Vacuum tu e,

two-way signal transmission systems. The exact nature and advantages of the inven- 5 An object of the invention is to improvethe tion Will be better understood from the following operating characteristics of long two-way signal detailed description thereof when read in connecing systems employing wave-controlled appation with the accompanying drawings in which: ratus for suppressing echoes and preventing sing- Fig. 1 shows diagrammatically a space dis- 0. ing. charge device having a feed=back circuit of the 10 Wave-controlled, circuit-control apparatus, series. type, such as S llS d n the transmission commonly called echo suppressors or anti-singing control circuits of the invention;

devices, are usually employed in connection with Fig. 2 illustrates graphically the operation of the repeating circuits of a long two-Way signalhe ed ck Ci cu OI F l;

ing system to insure that the system transmits in Figs, 3 and 4 show four-wire echo suppressors 15 only one direction at a time, so as to suppress embfldy diffe en OQifiCat Q S f h nv echoes, prevent singing, or both. This is usually tio nd accomplished by making the circuit-control ap- Fi 5 shows a s gnal-c d antin ng paratus responsive to signal transmission in device embodying other diiferent forms of the ineither direction over the system to increase or QIl OI 1 I 20 reduce the attenuation in the repeating paths.- 1 ShGWS a Simplified diagram Of a vacuum Certain circuits of the above mentioned type in be C c it h vi g a er es feed-back Such as is the prior art have been designed t rat i used in the transmission control circuits of the response to transmitted signals to properly vary inVentiOIln a th fi de uum th attenuation in the repeating paths by varytube 1 having a cathode or filament 2, an anode 2 ing the output impedance of a vacuum tube or p a and a c o electrode rid and tubes, connected in series or in parallel with the the u ua fi m heating, g d, bia ng and space repeating paths through transformers. By this cu en lllll bfil m fl s qwnh p t means the attenuation in the repeating path Quit ofv the tube 1 is coupled to the grid circuit transmitting signals is maintained small so that thfil e hrough t e feed-back transformer 5 30 it transmits the signals freely, while the attenuaving its primarywinding (i connected acrossan I ation of the oppositely directed repeating path is mpeqanse 3 9i, Va ue Z9 connected between the increased to the point where disturbing echoes p ate 3 9 tube 1 wi h? p imary Wind 9 or reflected currents are effectively suppressed. h u nu tra sformer m i the p at Q U o In such circuits, the maximum difierence in at- 11B tube a d itfiecondary w nding 1 connected 35 tenuation between the conditions of suppression d fies. between the-grid It; and the filament 2 of and transmission is limited by the ratio of inhe WW? I The secondary Winding of the outductance to resistance of the transformer wind-- D 11 transformer EQWJWQWQ $0 a! lead mu t ings, and the finite value of the vacuum tube f impeda ce.- Zs;

impedance, so that in very long transmission cir- I W ll be 35 -1 13; 21.

is the series 40 cuits the obtaining of the required amount of im- 91 18? connec on 9: the iced-Pact an me pedance change necessitates the use of unduly 5 a d 6 t e outputtransforme Ill 'Ifhe feedlarge transformers and vacuum tubes of very low a k; tl IlS iQl'IllH 5 i assumed to be an unequal output impedance, which is impracticable from 3410 dlfifimm havi g impedance Zn for an economic standpoint. ts primary w ding fi d Q 1 impedance Z for its In accordance with the present invention, a da y windina :The eedeloack transformer Wide difference in the amount of the attenuation 15 assumedto haYe, 8' voltage step-up of r. The introduced in the transmission paths between the cendarr winding ll o 01117121113 tr ns me 0 conditions of transmission and suppression is obas an im e an e 9f Z and its P i a y W n tained with echo suppressor circuits of the above 9 n mpedance o Z z. The impedance co-nmentioned type without resorting tothe use of nected ac oss thepr n a y W nd n 5 0f the feedunduly large transformers or vacuum tubes of back; transformer 5 will bedesignated' as Z0. particularly low output impedance, by utilizing Z12 is the mutual impedance between the self imwith the variable impedance vacuum tube or pedanc s Z1 and Z2 of the output transformer I0.

5 tu ssuiia l d si ned fe d-bac ci i s, The e. L t the vo t e mp fication f the ube. I be designated by m and its internal A. C. resistance by Re. Then the voltage E across the secondary winding of the output transformer III will be where i1 and i2 are the currents flowing through Z1 and Z2 respectively; and

'Zizi'r-l-(RoFZt-I-Zr-rfnZo) i2;='0 -(2 and therefore If the poling of feed-back transformer 5 is reversed, the sign of the rmZoj'term will change and the impedance looking into output transformer I becomes Equation shows that in the circuit of Fig. 1, as the feed-back is increased the impedance Z decreases and passes: through zero to a negative value. When the'impedance Zs of the external load is connected to Z, the vacuum tube circuit becomes oscillatory when Z is less than ZS. If the polingof feed-back transformer 5is reversed, as shown by Equation (6) the impedance Z increases with the feed-back. These various relations are shown graphically in Fig. 2 where 3 Equation (5) is plotted for positive and negative values of 1'. Although 1', being the turn ratio of the transformer 5, does not physically take on negative values, the expression"(rmZo) can be made positive or negative by the proper 'poling of the transformer 5. The graphical diagram of Fig. 2 indicates the possibility of obtaining zero impedance with a circuit "such as shown in Fig. l. Theoretically, the useful impedance change in the circuit of Fig.1. is from zero impedance to the open-circuit impedance of the. transformer I0. If a change from a theoretical zero impedance to the open-circuit impedance of the output transformer I0 is: desired, as would be the case in the echo suppressor circuitof Fig.-

'3, the tube I is made normally transmitting and on operation a rectified negative voltage is applied between its grid' and filament so as to annul the feed-back and make the impedance of tube I infinite. If a change from the open-circuit impedance 'ofjthe transformer II! to a theo-. retical zero impedance is desiredgas ,would be the case in the echo suppressor circuit'of Fig. 4, the tube I is made normally suppressed and a positive rectifiedvoltageis applied between its. grid andfilament to reenforce the feed-back and V suppressor ES. .In the echo suppressor ES, the

make the tube impedance-substantially zero.

Fig. 3 shows a series feed-back circuit,.s uch as illustrated in Fig.*1',-utilized in a four-wire'i echo suppressor of the series loss typeffor obtaining a wide difference in the amount of attenuation introduced iri' thesignal transmission paths between the conditions of transmission and suppression.

The circuit of; Fig. 3'comprises a one-way tran'sfi 'mission path EA including the-one-way ampliof the grid 4- of tube I more negative, so as to If the output transformer ,II I an ideal trans- "the manner well-known in the art.

Bridged across the path EA in the output of the amplifying device I2 therein is the input of an echo suppressor circuit ES comprising the space discharge-. amplifying device I4 and equivalent two-electrode space discharge rectifying device I5 .connectedin tandem by the transformer I6, and

a vaccum tube loss circuit I! having its output coupled effectively in series with the transmission path WA in the input of the amplifying device I3 therein, by output transformer Ill. The loss circuit'I Tcomprises a three-electrode vacuum tube with a series feed-back circuit, such as illustrated in Fig. l, the .corresponding elements of the circuits being identified with the same characters as used in Fig. L The output terminals of the rectifying device I5 are connected respectively to the terminals .of a condenser I8 which is connected in parallel with a resistance I9 in the grid-filament circuit of tube I in the loss circuit II. i

Another echo suppressor ES, which has circuit elements identical with those of echo suppressor ES just described, has'its input bridged across transmission path WA in the output of the amplifying device I3, and its output connected effectively in series with the transmission path EA inithe input of the amplifying device I2 therein as indicated in Fig. 3.

As both echo suppressor circuits ES and ES areidentical and operate in a similar manner, the operation-of the system of Fig. 3 will be clear from the following description of the operation of one echo suppressor, ES, only.

The circuit elements of tube I, in the loss circuit I'I, are chosen so that in the absence of a potential applied across the condenser I8 in the grid-filament circuit, tube I is in the transmitting condition- Thus, normally, the primary winding 9 of output transformer II] is effectively shortcircuited through the low impedance space path of tube I. The impedance Z inserted in the transmission path WA through the output transformer IIli will be quitelow (theoretically zero). In this case, the variable impedance tube I takes no power from the transmission path WA because it is made to supply its own losses by its feed-back circuit.

I When signals are being transmitted over the amplifying path EA, the main portion of the amplified signal wavesv in the output of the amplifying device I2 are transmitted directly to the listening subscriber. A small portion, however, of the amplified waves in the output of the device I2 are diverted into the input of the echo diverted signalv waves are further amplified by the amplifying device I4 therein and rectified by the rectifying device I5 coupled to the output of the device I4 by transformer I6. the amplifying device I4 and the rectifying device I5 are so arranged that the rectified signal voltagecharges the condenser I8 through resistance I9 in such direction as to make the potential The circuits of effective impedance inserted in series in the trans-.

mission path WA by the windings, of the output transformer ill is thereby increased to, its maxi! mum value, and by suitable choice of the values of the inductances of the transformer windings, this series impedance may be made such as to eifece tively suppress that path, preventing. transmission thereoverof echoes and reflected currentsdue to the signals transmitted over the path EA.

The resistance I9 is made large so that the charge on the condenser I8, and thus the blocking of the path WA will be maintained for a desired hangover interval after the speech signals cease flowing in t th EA. The hangover in the op-.

eration of the echo suppressor is requiredto al-. low for the time for the last wave of voice cure rent from the path EA to pass back over the path WA as an echo and arrive at the point where the latter path is blocked by the echo suppressor. 1 The speed with which the condenser l8 dis-&

charges, and consequently the effective length of this hangover interval, can be adjusted by change ing the magnitude of the resistance l9.

The four-wire echo suppressor circuit of Fig;

4 differs from that of Fig. 3 in that the output of the loss circuit H is coupled by the output transform-er It in shunt of the transmission path to be controlled instead of effectively in series I with that path as in the circuit of the latter figure.

This requires that in the normal condition the impedance looking from the transmission path to be controlled through the output transformer of the echo suppressor, be infinite in order that the transmission path WA or EA normally be operative to transmit with maximum eificiency. To accomplish this, an additional three-electrode bias reversing tube is inserted between the rectifier l5 and loss circuit I1, and tube in the loss circuit i1 is made normally inoperative by a large negative bias supplied to the grid circuit thereof by the voltage drop across resistance 2| in the grid circuit. The grid of the tube 20 is normally at zero potential, so that a high plate current flows through resistance 2|. The primary winding 9 of output transformer I0 is thus effectively opencircuited through the high impedance space path of tube When signal currents flow in the path EA, the rectified signal voltage in the. output of tube i5 is applied across the condenser l8 and parallel resistance l9 in the grid circuit of tube 23 in such direction as to render that tube inoperative. The plate current fiowing through re-- the transmission path WA through output trans former i9 is reduced to such a low value that the path WA is effectively short-circuited through the secondary winding ll of output transformer It in shunt therewith. The path WA is effectively suppressed by this short-circuiting connection when voice currents are being transmitted over the transmission path EA. As in the case of the echo, suppressor circuits, of Fig. 3, the series feedback circuit associated with the tube 1 enables a wide difference in the amount of attenuation introduced into the transmission path between the conditions of transmission and suppression to-be obtained without. necessitatin the use of unduly large transformers or control tubes of particularly low output impedance.

Fig. 5 shows the invention applied to an anti: singing circuit of the semi-neutral type suitable for use at the terminals of a radio telephone or, along cable. telephone system, for example, a,

submarine cable telephone system. It comprises a transmitting path TA including the one-way amplifying device- 22 for amplifying telephonic signals outgoing to the transmitting apparatus of the radio or cable system (not shown), and a receiving path RA including the one-way amplifying device 23 for amplifying signals received from the radio or cable terminal receiving appara-: tus.

Bridged across the transmitting path TA in the input of the amplifying device 22 through a transformer 24 are the control circuits 25 and 26.

Control circuit 25 has circuit elements identical with those of each echo suppressor in the circuit of Fig. 3, and control circuit 26 has circuit elements identical with those of each echo suppressor in the circuit of Fig. 4.

Control circuit 25 comprises the space dise charge amplifying device 21 having its input connected across the path TA by transformer 24, the equivalent two-electrode space discharge recs tifying device 28 having its input connected to the output of the amplifying device 21 by transformer 29, and a loss circuit 30, similar to the loss circuit H illustrated in Fig. 4, comprising a three- .electrode vacuum tube 3| with a series feed-back circuit, such as illustrated in Fig. 1. The input circuit of tube 3| includes a series condenser 32 and a resistance 33 in parallel therewith, the terminals of which are connected across the output terminals of the rectifying device 28. The output circuit of tube 3| is connected effectively in shunt of the path TA in the output of the amplifying device 22 therein by output transformer 34.

Control circuit 26 comprises the space discharge amplifying device 21 having its input connected across a transmission path TA by transformer 24, the equivalent two-electrode space discharge rectifying device 28 having its input connected to the output of amplifying device 21 by transformer 29, the three-eIectrOde-biaS reversing tube 35 including in its grid-filament circuit the series condenser 32 in parallel with the resistance 33, the terminals of which condenser and resistance are connected across the output terminals of the rectifying device 28, and the loss circuit 36 comprising the three-electrode space discharge tube 31 with a series feed-back circuit such as illustrated in Fig. 1, having its grid-filament circuit coupled to the plate-filament circuit of tube 35 by coupling resistance 38. The output circuit of tube 31 is connected effectively in shunt. with the receiving path RA in the input of the amplifying device 23 therein by output transformer 39.

Bridged across the receiving path RA in the output of the amplifying device 23 is the input of a control circuit 40 comprising the threeelectrode space discharge amplifying device 4| having its input connected across the path RA, the equivalent two-electrode space discharge rectifying device 42 having its input connected to the output of the amplifying device 4| by transformer denser 44 andparallel resistance 35 inseries'in the grid-filament circuit of the amplifying-device 2'! common to control circuits .25 and 25.

The tube 3| in loss circuit 33 is biased so as to be normally transmitting; and thus the primary winding of output transformert l is normally effectively short-circuited through the low .im pedance space path of tube 13!; The impedance looking from the path EA throughv the'output transformerB l is thereby'theoretically zero, sothatthe transmitting path EA isnormally short-- circuited through the secondary windingof output transformer 3 Therefore, it is seen that thecontrol .circuit 25 normally acts to disable the transmitting path EA in the output of amplifying device22. s The variable impedance .tube' 31 in loss circuit 36Tisnormally suppressed by the high. negative .biasfrom. the voltage dropacross resistance 38 in its "gridcircuit. The'grid. of. the. bias reversing tube 35 is normally at zero potential so thata high, plate current flows in resistance 38. Thus, the primary windingv of output. transformer 39 is effectively open-circuited through-the highv impedance space path of tube 37. Therefore, the impedancelookingfrom the-path RA through output transformer .39 is substantially infinite so thatthe control circuit 28 has little or no effect on the transmission of signals over thepath The operation of the system of Fig. will now be described. Let it be assumed that signal waves to. be transmitted are being sent over the transmitting path TA. These waves will be divided between the input of the a plifying device 22 andthe input of control circuits'ZEi and 2B. The portion divertedinto the control circuits 25 and 25 will be amplified-by the amplifying device 2'! and -rectifiedby. the rectifying device 28 common tothe, two control circuits. The rectified signal voltage inithe output of the rectifying device 28 will be. impressed across the condenser 32 and parallel resistance 33 onthe input "electrodes of thetube 3| in control-circuit 25, and on the input electrodesof the amplifying device 35 in control circuit 26, respectively. Therectifled signal voltcircuited throughthe high impedance space path of tube 3| connected between. its terminals. The

impedance looking from the path TA to the output transformer 3 is thereby increased from its normal theoretical zero valueto, such a high value as to effectively remove the short-circuiting disabling connection from that path. Thus, thecontrol circuit 25 is ineffective toplfeventtransmission of the amplified signalwaves from the output of the device 22 to theradio or cable transmitting apparatus. U

The rectified signal voltage across condenser 32 and parallel resistance .33 throws the grid of tube 35 so far negative as to render that tube inoperative... The plate 'currentof that tube being thus reduced to zero, the negative, potential ape,

plied across resistance 38 is removed leaving the grid of the variable impedancetube 3? at the fixed potential Ec supplied-.by its1-grid battery. The potential-EC is selected so that when it is alone applied, the. variable impedancejube .37, is rep-H;

' low impedance space path of tube 31.

and havinghts output'icoiinectedl across con dered operative over its most'efficientrangefi The primary' winding of output transformer 39 is thereby W effectively short-circuited through the This causes the impedance looking from the receiving path RA through the output transformer 39 to be decreased from" its normal infinite value to a value of zero theoretically, thereby inserting such a-loss in the path RA throughthebridging connection of the secondary winding of output transformer 39" thereto as effectively to suppress the path RA. Thus, incoming waves thereafter re-i ceived from the radio or cable receiving apparatus are prevented from reaching theinput of the amplifying device 23.' v

The resistance '33 is made of suitable value so that the charge OII thS condenser 32 is maintained for a desired hangover interval to prevent the transmitting path TA from being prematurely blocked and the receiving path RA from being prematurely? rendered operative whenthe supply of signalsto' control circuitsZE and 28 ceases.

:iPreferably 'receiving path RA should be sup-I pressed in response to' 'the signals transmitted cable transmitting apparatus. I To accomplish" this,I itm'ay be necessary to insert in the control circuit 25. a delay circuit or to'utilize other suit-' able 'means for' producing a lag in the operation of the losscircuit'30 with respect to that of the loss circuit-'35 in response to the rectified signal voltages impressed upon their respective inputs. To prevent 'aloss of part of the speech transmitted over the path TA, it may be necessary in some circuits to insert a delay'circuit of suitable design in that path between the point of connection thereto of the input of control circuits 25 and 26 and the point at which the output transformer 34 is connected thereto, to insure that the path TA is rendered operative before the amplified signals reach the point-in that path which is normally blocked by control circuit 25.

- The operation of the system, when signal waves arebeing received in thereceiving path RA from the radio or cable receiving apparatus, and outgoingsignalsare not being transmitted over the transmitting path TA, is as follows:

.The control circuit 26, being normally in such condition as to have no appreciable effect on the transmission of'waves over the path RA, the received-tsignalswillnbe transmitted to the ampli fying device .23 and amplified thereby. The main portion of the amplified signals in the output of the amplifying device 23 will be transmitted over the-:pathRA to'the listening subscriber. A portion of the amplified waves, however, will be diverted' into 'thecontrol circuit 46 and will be amplified in the amplifying device 4! therein. The amplified waves in the output of the amplifying device will be rectified by the rectifying devices? and impressed on the condenser 46 and parallel resistance 45, in the grid-filament circuit of" the amplifying device 21 common to the control circuits 25 and 26, in such direction as to throwthegrid .of the device 2'! so far negative as toprevent its operation by signal waves subsequentlydmpressed on the input circuit thereof fromithe path flf-A. 'Ifhus, subsequent false operation of control circuits 25 and 23 is prevented. Onv cessation of transmission .of received waves ilfilihe, pathRA, the condenser. 44 will discharge through the resistance 45 thus holding the ampli fying tube 27 inoperative for a definite hangover period,'which period may be adjusted to any de sired value by proper choice of the'value of the resistance d5. At the end of this period, the amplifying tube 2? will be returned to its normal condition so that it will be rendered operative by any subsequent speech waves transmitted over In the same manner as'explained in connection with Figs-l to 4, in accordance with the inven tion, the series feed-back circuits associated with tube 3! in'loss circuit 30, and with tube 37 in loss circuit 36, respectively, will cause a very wide difference in the amount of attenuation introduced in the transmission paths TA or RA respectively, between the conditions of suppression and transmission.

Certain tests which have been made indicate that as much as 40 decibels loss can easily be produced by circuits of the type shown in Figs. 1, and 3 to 5 inclusive in signal transmission paths, over a range from 200 to 3,000 cycles per second. This high loss is made possible by giving the feedback impedance, indicated as Z in Fig. 1, such a value that it will compensate for the non-ideal characteristics of the feed-back transformer and output transformer 3. A comparatively simple network was found to be sufiicient to cause as much as 40 decibels loss in a 600 ohm circuit.

It will be obvious that the general principles herein disclosed may be embodied in many other forms and arrangements without departing from the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. In a transmission system, a signal transmission circuit, an electric space discharge device having an input .and an output circuit, means inductively coupling the output circuit of said device to said transmission circuit so as to place an alternating current impedance in said transmission circuit which controls transmission therein, said impedance normally having a certain value, means for impressing a potential on said input circuit of said device so as to change said alternating current impedance in said transmission circuit to a different value, and means for effectively increasing the difference between the value of the normal alternating current impedance in said circuit and that of the changed impedance therein, the last mentioned means comprising a circuit for feeding back in proper phase relation energy from the output to the input circuit of said device.

2. In combination in a signal transmission system, a signal transmission circuit, an electric space dischargedevice having a cathode, anode and a control electrode, and circuits therefor, the cathode-anode circuit of said device being inductively coupled to said transmission circuit so as to place an alternating current impedance in said transmission circuit which controls transmission therein, said impedance normally having a certain value, means for applying a biasing potential to said control electrodesuch as to change the value of said alternating current impedance in said transmission circuit, and means for effectively increasing the difference between the normal and changed value of said alternating current impedance in said transmission circuit, comprising a circuit for feeding back in proper phase relation energy from the cathode-anode circuit of said device to its grid-cathode circuit.

3. The system of claim 2 and in which the feedcathodecircuit of said"device.-

back circuit is re OIlilCfGd with thef'grid 4. T e I system of claim 2" and in which "said alternating current'i'mpedance in; said transmis-' nected in series with the anode-cathode circuit and the grid-cathode circuit of said device.

'7. In a signal transmission system, two oneway transmission paths for transmitting in 0pposite directions, a three-electrode vacuum tube having its plate circuit effectively connected in series with one of said paths through a transformer, said tube being normally operative so that its plate impedance produces such a low loss in said one path that it transmits signals freely, means responsive to signals transmitted over the other of said paths to bias the grid of said tube to render the tube inoperative and thus increase the loss in said one path to the point where said one path is substantially disabled for transmitting signals, and means for increasing the difference between the values of impedance effectively inserted in-said one path in the operative condition and the disabled condition of said one path, the last mentioned means comprising a circuit for feeding back in proper phase relation a portion of the output energy of said tube from said plate circuit to the grid circuit of said tube.

8. In a signal transmission system, two oneway signal transmission paths for transmitting in opposite directions a three-electrode vacuum tube having its plate circuit effectively connected in shunt with one of said paths through a transformer, said tube being normally inoperative so that its plate impedance ,efiectively inserts in shunt with said one path an impedance of such high value that said one path transmits signals freely, means responsive to transmission of signals in the other of said paths for applying a biasing potential to the grid of said tube to render said tube operative and thus reduce the value of the impedance in shunt with said one path to the point where said one path is substantially disabled from transmitting signals, and means for cffectively increasing the difference between the values of impedance inserted in shunt with said one path in its operative and disabled conditions, the last mentioned means comprising a circuit for feeding back in proper phase relation energy from the plate circuit to the grid circuit of said 6 the impedanceinserted in shunt with the associated one-way path is of such high value as nor- ;mally to allow said other path to transmit signals free1y,means responsive; to transmission of signals-in the first one-way path to apply a bias-' ing potential to the grid of said one tube to render it "disabled r thus increasing the impedance effectively insertedthereby in shunt with said first pathto such a high value as to render said first .path tothe pointwhere' said otherpath is effectively; disabled, means responsive to signals transmitted over said other path to disablethe means for applying biasing potential 'to both tubes, and means for effectively increasing the difference between the valuesof the impedance inserted'in shunt with each transmission path by theassociated vacuum tubes in the operative and disabled condition of the path, the last mentioned means comprising a circuitv for feeding back in proper p aserelation energy from the plate circuit to the grid circuit of eachtube.

BJORN G. BJORNSON. 

